The issue of the boundary between The Republic of Texas and Mexico became an issue after 1845, when Texas was admitted to the U.S. as the 28th state. Mexico, which had never recognized the independence of Texas despite the fact they had lost a war and the support of the international community, immediately broke diplomatic ties with U.S. and refused to accept any type of offer for California.

The U.S. tried to buy the territory from Mexico, but they refused. The president of Mexico was actually deposed just for receiving the U.S. ambassador, who quickly left the country when he discovered how angry the Mexican people were at the time.

When the war finally broke out, there was much opposition from northern congressmen. Many of them realized that most of the land that was won during the war would be open to slavery, allowing the horrible institution to stretch from Atlantic to Pacific. Northerners tried to block pro-slavery forces by trying to pass the Wilmot Proviso, which would have closed all territory won in the Mexican-American war to slavery, but it didn’t pass congress.

Many of them also realized that the U.S. was acting like European monarchy by grabbing the territory of a nation whom the U.S. had sworn to protect just twenty years earlier.